Mono is adopted but has three siblings, but only knows about two. Said siblings are Harold, (shhh secret), and Mindless.

Mindless is technically the brother of Harold and (shhh secret), but ran away as a child and never found again, because of this, Mono was adopted as sort of a way for his parents to fill the hole of Mindless running away. Nobody actually knows Mindless’s identity and the brother of Harold and (secret) is presumed dead.